1. Field of the Invention
The present invention relates to a line-width measurement adjusting method applicable to line-width measurement of a pattern using a scanning electron microscope, or more specifically to a line-width measurement adjusting method and a scanning electron microscope capable of avoiding fluctuation in a measured value when changing magnification.
2. Description of the Prior Art
Measurement using a scanning electron microscope has heretofore been applied to a line-width measuring method of a pattern. Here, a scanning electron microscope is configured to scan an electron beam scanning range by irradiating incident electrons, to acquire secondary electrons emitted from a sample by using a scintillator, to convert a quantity of electrons thus acquired into luminance, and to display a surface image of the sample on a display device.
In the case of managing characteristics of a semiconductor device with use of this scanning electron microscope, it is a general practice to check whether or not a line-width of a pattern is formed in the size within a design standard. The management of the pattern line-width is typically executed in accordance with the following procedures. After displaying a predetermined range of a resist pattern formed on a semiconductor wafer on a display device, an electron beam is focused and irradiated on a measurement point in the displayed range to acquire a waveform of luminance distribution based on secondary electrons reflected from the measurement point. Then, a high-level width in the waveform of luminance distribution is determined as a line width. A judgment is made as to whether or not this line width falls within an acceptable error range. If the line width is within the acceptable error range, a subsequent etching process is executed. In contrast, if the line width is out of the acceptable error range, the wafer is sent back to a process for forming the resist pattern.
In this way, the line-width measurement of the pattern is important in the manufacturing process of the semiconductor device, and various methods for accurately measuring the line width have been proposed. In general, a position where a slope of luminance corresponding to a quantity of secondary electrons becomes the maximum is defined as an edge position of the pattern.
Meanwhile, as another technique related thereto, Japanese Patent Application Laid-open Publication No. Hei 5 (1993)-296754 discloses an edge detection method of determining, as an edge position, a position where a secondary electron signal becomes the minimum.
As described above, the line-width measurement of the pattern with a scanning electron microscope employs the method of determining the position where the slope of luminance becomes the maximum as the edge position or the method of determining the position where the secondary electron signal becomes the minimum as the edge position.
Here, there are cases where measurement is performed by using the magnification of observation of different numbers of times, and where measurement is performed with the beam diameter of the electron beam in different size when a scanning direction or a measurement device is changed. These cases may cause a problem that different measurement results are obtained from the same measurement object.
When rescaling the magnification of observation, the measurement results may vary as follows. For example, use of the high magnification of observation causes an electron beam to travel in a short distance per unit time as compared to the case of use of the low magnification, whereby a range irradiated with the electron beam for a certain period of time is reduced. Generally, the detected edge position changes according to the size of the range irradiated with the electron beam, and this produces differences in the measurement result of the line width. For this reason, the measurement result of the line width varies between the case of high magnification of observation and the case of low magnification of observation.
Meanwhile, changes in the beam diameter causes the range irradiated with the electron beam to also vary. As similar to the case of resealing the magnification of observation, this also produces differences in the measurement result of the line width even when the same line width is measured.
In order to deal with these problems, a countermeasure has been taken heretofore in which the observation error is corrected by firstly acquiring correction data on the line width in each scale of the magnification of observation, and then by using the correlation in the correction data. Moreover, in the case of the changes in the beam diameter, the line width of the same sample is measured either in different scanning directions or with different measurement devices, and then beam diameter is readjusted when the measured values are different. However, it is troublesome to prevent the measurement results of the line width of the same pattern from varying, and difficult to execute the line-width measurement process efficiently.